Single-photon avalanche photodiodes (SPADs) are becoming of wide use in many domains, such as nuclear medicine, radiation detection in safety and security, biology and Lidar. The SPADs can be produced in custom micro-fabrication technology as well as in standard CMOS. The first approach allows a technology development aimed at optimizing the sensor performance in terms of efficiency and noise. The second approach allows the integration of intelligence directly on the photo-sensor.
A very interesting solution merging the benefits of the two above-mentioned technologies is offered by the high-density three-dimensional (3D) interconnection, which is widely used in image sensors. Each SPAD of the array can be read-out independently with a connection placed opposite to the light entrance side. A challenging aspect of the 3D approach is to provide an independent SPAD contact access opposite to the illumination side. Currently, there are mainly two approaches: Through Silicon Vias (TSVs) and back side illumination (BSI). In the first case (TSV), conductive pillars going from one side to the other are placed close to each element bringing the independent front side electrode to the back. The pillars are isolated from the silicon surrounding them. In the second case (BSI), the entrance window is placed opposite to the structured side.
The TSV approach requires many technological steps and it usually causes a loss of sensitive area, especially for small SPADs. BSI requires a complete redesign of the current SPAD technologies and could lead to some drawbacks such as increased optical cross-talk and loss of detection efficiency.